Method of inducing polarization of active magnesium surfaces

ABSTRACT

Methods for surface polarizing magnesium and its alloys; thus permitting large pieces to be readily anodized using only nominal electric power. Low adhesion, easily strippable film strips are adhered to the pieces, the strips providing areas of exposed metal which allow a high current density to be built up thereat which produces vigorous electrolytic action for progressively peeling off the films for subsequent anodization of the pieces.

United States Patent Reyburn Wick Southampton, Pa.

May 12, 1970 Nov. 16, 1971 The United States of America as representedby the Secretary of the Army Inventor Appl. No. Filed Patented AssigneeMETHOD OF INDUCING POLARIZATION OF ACTIVE MAGNESIUM SURFACES 8 Claims,No Drawings US. Cl 204/56 M, 204/ l 5 Int. Cl C23b 9/00, C23b 5/48 Fieldof Search 204/15, 56 M [56] References Cited UNITED STATES PATENTS2,214,876 9/l940 Clark 2,348,826 5/l944 Krause et al.

2,880,148 3/l959 Evangelides Primary Examiner-John H. Mack AssistantExaminer-T. Tufariello 204/56 M 204/56 M 204/56 M Attorneys-Harry M.Saragovitz, Edward J. Kelly, Herbert Berl and S. Dubroff sequentanodization of the pieces.

METHOD OF INDUCING POLARIZATION OF ACTIVE MAGNESIUM SURFACFS Theinvention described herein may be manufactured, used, and licensed by orfor the Government for governmental purposes without the payment to meof any royalty thereon.

This invention relates to the anodization of magnesium and magnesiumalloy pieces, and more particularly concerns improved methods for theiranodization by inducing polarization at newly exposed specified surfacesthereof.

A difficulty sometimes encountered in anodizing magnesium and its alloysin the past resided in their inability to polarize. With high-voltagealkaline processed, required current density may readily be attained,but voltage will fail to rise to proper operational range, i.e., fromabout 60 to 100 volts. lfnot corrected, operation under low voltageconditions will result in formation of an undesirable thin film, ratherthan a normal thick coating.

Another practice employed was to apply a high source of current to themagnesium work pieces such that current density is momentarily increasedapproximately three times the nonnal value. In most instances,anodization would proceed normally thereafter. In a few instances, arepetition of the surge would be necessary. If the amount of the workbeing processed however required the entire capability of the powersupply, there would be no reserve capacity to supply the needed surge ofcurrent.

In such cases, it was necessary to reduce the size of the load in thetank such that present current capacity would yield the desired surgeload. Such reductions obviously lengthen process time with obviousundesirable effects. An alternative would be to divide the work lotsinto sufficiently small groups partially anodize each, then rejoin themfor the final portion of anodizing. In the case of very large pieces, itis apparent these procedures described above would not successfully beutilized.

Such large parts may be allowed to oxidize in the air for several days,after which time they may be reanodized with an increased chance ofproper polarization occurring.

It is therefore an object of this invention to provide improved methodsfor anodizing magnesium and its alloys.

Another object of the invention is to provide methods wherebypolarization if newly exposed magnesium surfaces is readily induced.

Still another object of the invention is to provide methods wherebylarger magnesium or magnesium alloy pieces may be efficiently andrapidly anodized with a nominal supply of electric power.

These and other objects of the invention will be apparent to thoseskilled in the art upon studying this disclosure.

ln accordance with the aforementioned objects, I have discovered that ahigh current density will result at predetermined portions of amagnesium piece to thereby efiect immediate polarization at thoseportions.

More specifically, l have discovered that selected areas of magnesiumpieces may be masked off with a strippable film having poor stopoffqualities or a low adhesion to the magnesium or magnesium alloy, tothereby effect polarization of those surfaces not so masked. Theanodizing process may then be started and the current adjusted to thatrequired for the entire area of the workpieces, and resulting in a highcurrent density begins applied to the exposed magnesium surfaces whichthen become polarized. Within a few seconds, the vigorous electrolyticaction occurring adjacent the strippable film edges will roll the filmback to expose more bare metal. Because of the low resistance of thefreshly exposed metal, a larger amount of current passes thereat,resulting in a higher density. As the film peels progressively, thishigh current density area follows until the entire film drops off.Anodizing then proceeds normally over the entire piece.

For maximum effectiveness, the film should be applied in bands or stripsaround the workpiece, about an inch or so wide, the bands beingapproximately one-sixteenth to onefourth inch from each other. The filmmaterial is preferably a plastic coating compound, strippable, havingpoor stopoff properties, and is described in Military SpecificationMlL-P-4 502"), Mar. 18, I969. It comprises:

Cellulose ucetate butyrute'. (maximum free acidity, ul acetic acid0.02%) 56 g. Chlorinated biphenyl 52 g. Polyethylene glycoldi-2-ethyl-hexunoate l2 Mineral oil 8 g. Xylol I67 ml. lmpropunol 82.3ml. Phenylsulicylate l g. Tn'lauryl phosphate 1 g. Di-2-ethyl hexylamineor methyl morpholine 1 ml. Solubilized l0% copper-B-quinolinolate 0.75ml.

The preservatives and fungicides called for in the MilitarySpecification need not be included for these purposes.

My invention is not limited to the above strippable film compound, anyplastic film compounds having poor stopoff 20 properties on magnesium orits alloys being generally satisfactory.

The thickness of the film will generally be at least about 0.005 inches.Preferably, the film should be of sufficient thickness to mask the metalsurface such that when progressively peeled by the polarizing action, itwill remain as a continuous piece and not become flaky. The inventiontherefore contemplates thicknesses up to about one-eighth inch thick.

The film will normally be placed along or around the piece in stripsabout 1 inch wide and will provide spaces therein- 3 between rangingfrom about one-sixteenth to one-fourth inches wherein the magnesium ormagnesium alloy piece remains exposed. Since the normal range of currentdensities is about 15-30 amperes per square foot (hereinafter referredto as a.s.f.), it will become necessary for larger pieces to be maskedoff to a proportionately greater extent, or to reduce the area of theexposed metal, in order to achieve desired current densities. The powersource cannot and need not differentiate between one large piece orseveral smaller pieces having surface areas.

The strips or bands may be applied to the metal piece by brushing,dipping or spraying techniques. When brushing the strippable filmmaterial onto the workpiece in accordance with my invention, thematerial can ordinarily be used as is. If dipping is to be employed, itis desirable that the film material be heated to about l001 10 F. Whenspraying is contemplated, the viscosity of the film material should belowered with solvents such as xylol, isopropyl and isoamyl alcohols.When dipping or spraying techniques are used, it will be 50 necessary toremove portions of the film by a knife or razor to expose the baremetal. Regardless of the means of application, the film should beallowed to dry for about 24 hours at room temperature.

The upper limit of the size of the workpiece to be anodized is governedby the electric power available. The workpiece contemplated for use withmy invention comprises magnesium or magnesium alloys such as AZ3l, A291,and the like, wherein magnesium will usually comprise at least about 90percent of the alloy, although alloys having less than this amount havebeen induced to polarize readily. The shapes and configurations of theworkpieces are necessarily limited to those which lend themselves to theorderly reception of the strippable film thus eliminating fromconsideration very intricate and hollow shapes.

In order to more clearly define my invention, the following examples areherein presented for illustrative purposes:

EXAMPLE 1 Two 4X6XV4-inch plates of AZ9l were prepared for anodizationin accordance with Military Specification MlL-M-45202, dated 20 Nov.1959, paragraph 5. The plates were then brushed with the aforedescribedstrippable film material along its length in 4 strips of equal widthsuch that approximately percent of the piece was masked, or 15 percentof the surface area was exposed. The film thickness was approximately0.04 inches. A variable AC transformer having a l ampere maximum outputwas employed. The initial cur rent density was 135 a.s.f. and thevoltage ranged from 0-80 AC After the strippable film was progressivelypeeled and removed, the current density was 15 a.s.f. An anodic film 15mils thick resulted after 90 minutes of operation. The bath compositionused for this and succeeding examples is specified in MilitarySpecification MIL-M45202, dated 20 Nov. 1959, Type ll, class A(High-Voltage Alkaline Process), as well as the tank equipment.

EXAMPLE ll Two identical plates as described under ex. 1 were sprayedwith the aforedescribed strippable film and allowed to dry over night.One pint of the strippable film material was thinned with 20 ml Xylol, 5ml isopropyl alcohol and ml. isoamyl alcohol. The film was carefullyremoved by razor blade from selected portions of the workpiece such that5 spaced-parallel areas of exposed metal resulted each beingapproximately inch wide. The film thickness was approximately 0.025inches. A variable AC transformer having a 10 ampere maximum output wasemployed, the initial current density being approximately 130 a.s.f. andthe voltage ranging between about 0-80 AC After the strippable film wasremoved, the current density was 18 a.s.f. An anodic film of 16 milsthick resulted after 87 minutes of operation.

EXAMPLE [I] Two l2Xl2X /4-inch AZ3I plates were prepared as describedunder ex. 1. The plates were brushed with the strippable film compoundaforedescribed such that l l spacedparallel stn'ps covered about 90percent of the entire workpiece surfaces. The film thickness wasapproximately 0.045 inches. A variable transformer having a maximumoutput of 30 amperes was employed. The initial current density wasaround 145 a.s.f. and the voltage ranged between 0-85 AC After thestrippable film was removed, the current density was a.s.f. An anodicfilm 1.5 mils thick resulted after 85 minutes of operation.

lclaim:

1. A method for inducing polarization of newly exposed specifiedsurfaces of magnesium and magnesium alloy pieces prefatory toanodization thereof comprising the steps of adhering a poor stopoffstrippable plastic film along selected portions of said pieces, saidfilm providing narrow-spaced sections wherein surfaces of said piecesare exposed to form treated pieces having substantially alternatingmasked and unmasked surfaces,

electrically connecting said treated pieces in an anodizing tank filledwith anodic bath composition used in highvoltage alkaline processes,adjusting the current to that required to anodize the entire surfacearea of the pieces, said adjusted current forming high current densityat the exposed surfaces of said treated pieces,

maintaining said current until vigorous electrolytic action occurs atthe exposed portions of said treated pieces and edges of said strippablefilm to continuously progressively roll back those portions of the filmadjacent the electrolytic action and until all film is stripped fromsaid pieces,

further maintaining said current until said pieces are anodized.

2. The method as described in claim 1 further characterized by cleaningsaid pieces prefatory to adhering said strippable films thereto.

3. The method as described in claim 1 wherein adhering said strippablefilm comprises the step of brushing spaced bands of film material ontosaid pieces.

4. The method as described in claim I wherein adhering said strippablefilm comprises the steps of dipping said pieces into strigpable filmmaterial and then provi mg spaced areas of exposed metal on said piecesby removing selected portions of said dipped film.

5. The method as described in claim I wherein adhering said strippablefilm comprises the steps of spraying said pieces with strippable filmmaterial and then providing spaced areas of exposed metal on said piecesby removing selected portions of said sprayed film 6. The method asdescribed in claim 1 wherein said strippable film comprises:

Cellulose acetate butyrate; (maximum free acidity. us acetic acid 0.02%)56 g. Chlorinated biphenyl 52 g. Polyethylene glycoldi-2-ethyl-hexanoate l2 g. Mineral oil 8 g. Xylol I67 ml. lsopropanol82.3 ml. Phenylsalicylate l g. Trilauryl phosphate I g. DiZ-ethylhexylamine or methyl morpholine l ml. Solubilized 10%copper-8-quinolinolate 0.75 ml.

7. The method as described in claim 1 wherein said magnesium alloy isselected from the group consisting of A231 and A291.

8. The method as described in claim 1 wherein said masked surfacescomprises about to percent of the total surface area of said pieces.

* t i III

2. The method as described in claim 1 further characterized by cleaningsaid pieces prefatory to adhering said strippable films thereto.
 3. Themethod as described in claim 1 wherein adhering said strippable filmcomprises the step of brushing spaced bands of film material onto saidpieces.
 4. The method as described in claim 1 wherein adhering saidstrippable film comprises the steps of dipping said pieces intostrippable film material and then providing spaced areas of exposedmetal on said pieces by removing selected portions of said dipped film.5. The method as described in claim 1 wherein adhering said strippablefilm comprises the steps of spraying said pieces with strippable filmmaterial and then providing spaced areas of exposed metal on said piecesby removing selected portions of said sprayed film.
 6. The method asdescribed in claim 1 wherein said strippable film comprises: Celluloseacetate butyrate; (maximum free acidity, as acetic acid 0.02%) 56 g.Chlorinated biphenyl 52 g. Polyethylene glycol di-2-ethyl-hexanoate 12g. Mineral oil 8 g. Xylol 167 ml. Isopropanol 82.3 ml. Phenylsalicylate1 g. Trilauryl phosphate1 g. Di2-ethyl hexylamine or methyl morpholine 1ml. Solubilized 10% copper-8-quinolinolate 0.75 ml.
 7. The method asdescribed in claim 1 wherein said magnesium alloy is selected from thegroup consisting of AZ31 and AZ91.
 8. The method as described in claim 1wherein said masked surfaces comprises about 80 to 90 percent of thetotal surface area of said pieces.